1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly to a reflective LCD device including a cholesteric liquid crystal (CLC) color filter.
2. Description of Related Art
In general, LCD devices are divided into reflective LCD devices and transmissive LCD devices. The transmissive LCD device uses an internal light source such as a back light, while the reflective LCD device uses ambient light.
Particularly, since the reflective LCD device uses ambient light, the brightness of the display depends on circumstances. In an office, the reflective LCD device is lower in brightness than the transmissive LCD device and, accordingly the color purity of an absorption-type color filter used in the LCD should be sacrificed to increase the brightness.
FIG. 1 is a cross-sectional view of a conventional reflective liquid crystal display.
As shown in FIG. 1, the liquid crystal panel includes a linear polarizer 26, a retardation film 24, a diffuser film 22, a first substrate 10, a color filter 20, a common electrode 18, a liquid crystal layer 16, a reflective electrode 14 and a second substrate, each are stacked in the above-described order.
The reflective electrode 14 reflects light transmitted from outside the display and also functions as a pixel electrode. The reflective electrode 14 and the common electrode 18 apply a voltage to the liquid crystal layer 16 and change the orientation of liquid crystal molecules. The diffuser film 22 reduces a surface reflection of light and increases a viewing angle. The retardation film 24 such as a λ/4 plate converts linearly polarized light into circularly polarized light. Further, the linear polarizer changes the natural light into linearly polarized light.
The reflective LCD device described above functions and acts as follows.
When natural light is incident into the LCD device, the natural light is converted into linearly polarized light by the linear polarizer 26, then converted into circularly polarized light by the retardation film 24. The circularly polarized light is converted into linearly polarized light while passing through the liquid crystal layer 16 and is reflected on the reflective electrode 14. The reflected polarized light is converted into circularly polarized light while passing through the liquid crystal layer again, then passes through the color filter to produce colored light.
The circularly polarized light is diffused to increase the viewing angle while passing through the diffuser film 22, then is converted again into linearly polarized light while passing through the retardation film 24. The linearly polarized light is displayed to the user after passing through the linear polarizer 26 in the form of images.
FIG. 2 shows the state of light while it passes through each of the components described above when an electric field is not applied to the liquid crystal layer.
The natural light is first converted into linearly polarized light through the linear polarizer 26. The linearly polarized light is changed into circularly polarized light through the retardation film 24. The circularly polarized light is converted again into linearly polarized light through the liquid crystal 16, then reflected by the reflective electrode 14. The reflected linearly polarized light is changed into circularly polarized light through the liquid crystal layer 16. The circularly polarized light is finally converted into linearly polarized light through the retardation film 24.
FIG. 3 shows the state of light while it passes through each of the components described above when an electric field is applied to the liquid crystal layer.
The natural light is first converted into linearly polarized light through the linear polarizer 26. The linearly polarized light is changed into circularly polarized light through the retardation film 24. The circularly polarized light is not changed when passing through the liquid crystal 16 as an electric field is applied to the liquid crystal 16, then reflected by the reflective electrode 14. The reflected circularly polarized light is not varied even when passing through the liquid crystal 16. The circularly polarized light is finally converted into linearly polarized light through the retardation film 24, then absorbed by the linear polarizer 26.
FIG. 4 is a graph illustrating the reflectivity of light with respect to the incident light of the LCD device described above. In FIG. 4, the X-axis indicates a wavelength λ, and the Y-axis indicates a reflectivity. Note that a dominant wavelength region is referred to as region A and other wavelengths are referred to as region B. As shown in the graph, though light's reflective index is relatively high in the region A, because light reflection is also carried out in the region B, the color purity of the LCD is reduced. It is required that the color purity is reduced in order to increase the transmissivity of the color filter, but just lowering the color purity to increase the brightness has a limitation.
Further, since the LCD having the configuration described above has a multi-layered structure in which each layer, i.e., each component differs from one another in reflective index, the intensity of the light is reduced while the light passes through each component. For example, the intensity of the light first is reduced while passing through the linear polarizer 26, and then also prominently is reduced after passing through the color filter 20, because part of the light is absorbed or reflected while passing through the color filter 20.
Further, though the observer can clearly see the image displayed due to a good contrast ratio in the center of the screen, the contrast ratio becomes lower as it gets far from the center of the screen, thereby deteriorating the display characteristic.